Most homes, offices, and other dwellings have some type of heating system which includes some sort of control system that controls the "on" and "off" operation of the system. Although a wide variety of heating systems exist today, most involve the combustion of a fossil fuel as the energy source.
Oil based heating systems typically employ a motor/fuel pump to provide a combustible fuel/air mixture and an igniter to provide a spark to ignite the mixture. Conventional systems generally employ a switching device to connect an energy source to the igniter and motor in response to a call for heat from a thermostat.
Conventional switching devices utilize a relay. When the thermostat senses a temperature below the desired or set temperature, it closes and causes the relay to be energized. When the thermostat senses the preset temperature, the relay is de-energized, thereby disconnecting the igniter and motor from the energy source.
Although relays provide a convenient way of controlling application of electrical energy to the igniter and motor, they are susceptible to a variety of problems. In particular, the continuous "on" and "off" cycling of the heating system combined with power loading across the movable contact interface, may result in the movable contact being welded "closed," thereby rendering its relay inoperable to shut "off" the igniter and/or motor.
To overcome this problem, some switching systems are designed with two relays (which may be combinations of electro-mechanical or electronic relays) configured in series. In theory, if one relay becomes welded, the other relay would still function to turn "off" the igniter and motor when the thermostat is opened. However, this solution is at best a temporary one in that the system would continue to operate and because both relays have sustained wearing action, it is likely that the second relay contact will become welded at a subsequent heat cycle.
Conventional control systems are also designed such that the igniter and motor are turned "off" if no flame is detected after a certain period of time, commonly referred to as the trial for ignition (TFI) or start-up period. Such systems generally utilize a photocell adapted to detect a flame in the combustion chamber, and if no flame is present after the TFI period, the relays will be de-energized and the system is "locked-out" by activation of a lock-out circuit. Thereafter, the system can only be operated by manual activation of a reset switch. Photocells are typically adapted to operate in the thermal radiation region of the electromagnetic spectrum.
Use of an igniter in conjunction with a photocell that detects thermal radiation, presents a unique problem in that at start-up, the igniter gives off radiant heat which may cause the photocell to become activated before any combustion. If this occurs, the motor will continue to pump fuel into the combustion chamber which may lead to dangerous conditions. To overcome this problem, conventional flame sense circuitry is designed such that at start-up, it compensates for the additional radiant heat provided by the igniter. After the start-up period, the flame sense circuitry returns to a higher sensitivity level so that it can properly detect when the flame goes out.
Conventional flame sense circuitry, however, has several drawbacks. First, conventional circuitry is relatively imprecise in providing control signals indicative of whether "flame" or "no flame" exists. This is a disadvantage in that if a flame occurs at the end of the TFI period, the system may be "locked-out" unnecessarily due to the imprecise output of a control signal indicative of flame.